Best prop for 1976 Johnson 15 top end speed?

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Ghostman3

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I have a 9.25x10 prop from evinrude. If I want to decrease the prop size for a higher top end speed for a heavy boat, what size prop should I get?

Background: When I modified the outboard from a 9.9 to a 15, I only gained about 2 mph. If I am overpropped to take advantage of the higher rpms, should I try an 8x10 prop or something else?
 
First off....what year 9.9 did you modify and what parts did you change to modify it?
The reason I ask is that different years require a different mix of parts to make the engine a true 15hp.
Second...as the previous poster advocated........you need to establish a base line RPM before throwing props at it unless you have a supply of them around.
 
Like everyone has mentioned before, you'll need to find the rpm's you are turning now before you can figure out what size you need to go to. You might also need to find the weight of the craft you are pushing to help as well.

My 1993 Johnson 15hp had a 9.25x12 prop when I bought it and was way too big for my 16ft Tracker Sportsman(I could hear the rpm's were not even close to what the should be), I previously used my brothers 1986 Johnson 15hp on it so I had a little something to compare it with. By big I am referring to the pitch of the prop. I dropped down to 9.25x10 and gained around 5-6mph(17.5mph). It was fine until I had another person or two ride with me, so I dropped down another prop size(9.25x9), when I did and gained a little quickness on takeoff but speed maintained the same with just me and I am now able to take 1-2 other ppl with me and not loose more then 5-6mph, depending where they sit and how much more junk they bring then me. LOL I don't know everything but in my head the 9.25x9 is the best prop for my boat/motor and how I use it. By myself I am probably at the upper end of the rpm range and with 2 extra ppl I am on the lower end of that range. I am however considering going down another prop size to throw on only when I know I will have 2-3 ppl total in my boat all week. I really should have a tach to test it though!

I hope my experience will help you,

Al
 
You have your prop numbers mixed up. Diameter is the fist number and pitch is the second number. You would want to lower the pitch,2nd number to increase your rpm's. It is roughly +/- 200 rpm's per inch of pitch.
 
I had the boat out today and the according to the timing light I was going at mostly 5100 RPM at full throttle. This gives me some room to play with, but what is the ceiling I should not go past? 5500 RPM or 6000 RPM. Also how far can I decrease the pitch for the prop and be safe? I currently have a 9.25x10 prop.
 
I've got a 1978 15hp ,the original prop is 9 1/4 X 8 in very good condition ,with the 14 ft fully loaded, trolling motor, batt, cooler, gear & 2 people it will push it 19 mph according to the GPS. Brought a 91/4 X 11 prop thinking it would go faster but I could tell it was too much prop and motor was bogging down not reaching proper RPM's .Didn't have the GPS with me at that time so not sure how much speed I lost. Got a tiny tach on order to get it right, but I would think that you would like to keep the RPM's between 4900 to 5500 just my 2cents
 
Ghostman3 said:
I had the boat out today and the according to the timing light I was going at mostly 5100 RPM at full throttle. This gives me some room to play with, but what is the ceiling I should not go past? 5500 RPM or 6000 RPM. Also how far can I decrease the pitch for the prop and be safe? I currently have a 9.25x10 prop.
Click to expand...
When you say you modified it to a 15 I am curious as to exactly what you did to it. 5200 RPMs would be pretty much your maximum you would want to turn it anyway. You don't want to turn it all the way up to the max allowable because you are playing with fire there.
 
I had a spec sheet at one time for my old '82 OMC 15 hp. The max RPM was 7000 if memory serves me correctly. And that is assuming you have a true 15 hp motor; meaning ALL the mods were done (not just the carb). Last time I had mine in the water (about 4-5 years ago), it ran 7000 RPM from the no-wake zone to the fishing hole; about 2 miles. Then same thing all the way back. Its made that trip a bunch of times. It doesn't seem to hurt it. Good little motor; I just don't use it anymore now that I have the little 25 Yamaha. That 15 used as much gas as the 25 does (25 hp 4 stroke) and makes twice the noise; after I drown in my own exhaust smoke.
 
turbotodd said:
I had a spec sheet at one time for my old '82 OMC 15 hp. The max RPM was 7000 if memory serves me correctly. And that is assuming you have a true 15 hp motor; meaning ALL the mods were done (not just the carb). Last time I had mine in the water (about 4-5 years ago), it ran 7000 RPM from the no-wake zone to the fishing hole; about 2 miles. Then same thing all the way back. Its made that trip a bunch of times. It doesn't seem to hurt it. Good little motor; I just don't use it anymore now that I have the little 25 Yamaha. That 15 used as much gas as the 25 does (25 hp 4 stroke) and makes twice the noise; after I drown in my own exhaust smoke.
Click to expand...
I have had 3 over the years the newest being a 78 all of them being factory 15s and all of them WOT was 5500 RPMs according to the manual. I still have the '78 manual here and just looked at it. They may have increased them more as they got newer but I was pretty sure the older ones didn't turn up that much.

That was why I was saying 5200 should be tops.
 
I put on a carb from a 1977 15 hp. According to Leroy's Ramblings the only difference at the time was the carb. The tuned exhaust did not happen until latter. The plate on the motor which has the model and serial numbers state an RPM range of 4500-5500 for the 9.9. I would assume for a 15 it would be higher. After researching Leroy's, 15 hp should have a operating range of 5500 to 6500 rpms. The Evinrude website sells props with diameters between 7 and 11. If I'm at 10 and go down to an 8, it should increase my rpms to about 5500. Even if it goes to 6000 I should be ok. If it starts getting really out of wack then I'll worry. I do want to thank everybody for their input especially in having to go and test the outboard to find out what the current rpms are.
 
nccatfisher said:
turbotodd said:
I had a spec sheet at one time for my old '82 OMC 15 hp. The max RPM was 7000 if memory serves me correctly. And that is assuming you have a true 15 hp motor; meaning ALL the mods were done (not just the carb). Last time I had mine in the water (about 4-5 years ago), it ran 7000 RPM from the no-wake zone to the fishing hole; about 2 miles. Then same thing all the way back. Its made that trip a bunch of times. It doesn't seem to hurt it. Good little motor; I just don't use it anymore now that I have the little 25 Yamaha. That 15 used as much gas as the 25 does (25 hp 4 stroke) and makes twice the noise; after I drown in my own exhaust smoke.
Click to expand...
I have had 3 over the years the newest being a 78 all of them being factory 15s and all of them WOT was 5500 RPMs according to the manual. I still have the '78 manual here and just looked at it. They may have increased them more as they got newer but I was pretty sure the older ones didn't turn up that much.

That was why I was saying 5200 should be tops.
Click to expand...
 
nccatfisher said:
turbotodd said:
I had a spec sheet at one time for my old '82 OMC 15 hp. The max RPM was 7000 if memory serves me correctly. And that is assuming you have a true 15 hp motor; meaning ALL the mods were done (not just the carb). Last time I had mine in the water (about 4-5 years ago), it ran 7000 RPM from the no-wake zone to the fishing hole; about 2 miles. Then same thing all the way back. Its made that trip a bunch of times. It doesn't seem to hurt it. Good little motor; I just don't use it anymore now that I have the little 25 Yamaha. That 15 used as much gas as the 25 does (25 hp 4 stroke) and makes twice the noise; after I drown in my own exhaust smoke.
Click to expand...
Click to expand...


I have a 95 model 15Hp that is also rated at 7000 rpms. I run a 3 balde 8" pitch aluminum prop at about 6900 rpms when solo and have never had any problems. It is noisy but it pushes my 1648 near 20mph and has great holeshot. I was going through the trouble of putting on a 10" prop for when I was solo and saving the 8" for a load and with another passenger but I've since quit wasting time. The 10" would only run 1 mph faster (maybe 2 on glass water) and the rpms only dropped to 6500 so didn't really save on gas or anything either.

To the OP, your rpms seem low unless you are in a heavy boat. I have an 82 9.9hp that will run about those same numbers with a 10" prop. Just swapping to the 15hp carb only got me 300-400 more rpms. Also have a 82 true 15hp and it will run about 6200 rpms with the factory 10" prop on the same hull and load.
 
Thank you for that bit of information. I think I can raise the motor another 1/2 inch and that will get me in the higher rpm range. My transom on the old boat is only 15 1/2 " and the motor is right at 17 so along with the prop I've ordered a Jack plate to raise it the extra it needs and will be in tomorrow late afternoon.
 

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